In the U.S., railroads replace over twenty million ties per year. The service life of solid wood ties is diminished because of inadequate resistance to compression and impact stresses, resulting in mechanical damage to the tie at the point of tie plate contact and loosening of the spikes due to compressive failures. Also, adequate penetration of preservative into a tie is difficult and, when checking occurs in service, a means of invasion by water and micro-organisms is provided to the under-treated tie interior. Still further, the trend to larger ties to withstand increasing traffic axle loads is hampered by reduced premium timber resources.
Several attempts have been made to develop ties of improved serviceability by consolidating comminuted wood. Three advantages of doing so are readily apparent:
1. densities are easily attained which eliminate compression damage, PA0 2. preservative may be uniformly distributed on the wood before consolidating, and PA0 3. uniformity of wood with absence of knots, checks or other defects to minimize rejects and in service failures.
Thus, commercial quarter inch hardboard laminated with glue to the usual tie thickness has been extensively studied but not adopted. One reason being the cost of the large glueline areas is substantial.
A tie prepared by consolidating the full thickness in one unit eliminates the glueline expense, but encounters a slow-to-heat mass and associated manufacturing problems, along with possible beam strength deficiencies.